Ferrari's Fernando Alonso set the pace in a wet-dry first practice session at the Belgian Grand Prix.
The Spaniard edged the two Force Indias of Paul Di Resta and Adrian Sutil when the track was at its driest about two-thirds of the way through the session.
McLaren's Sergio Perez was fourth, ahead of Mercedes' Nico Rosberg and Red Bull's title leader Sebastian Vettel.
Mercedes driver Lewis Hamilton was briefly fastest as the track was drying but ended the session 15th.
The session started on a damp track and the rain returned in the final 15 minutes, leaving only a short period when the circuit was in the best condition.
The times were not representative of the competitive order that can be expected through the weekend - Alonso's fastest lap was one minute 55.198 seconds, still several seconds off the times that would have been set on a fully dry track.
Di Resta was just 0.026secs behind the Ferrari and Sutil 0.149secs further back. Rosberg was 0.416secs off the pace and 0.022secs ahead of Vettel, who had a spin at La Source when exploring the damp track.
"The times will be a lot closer when the running gets more serious than they are at the moment," BBC 5 live analyst and former F1 driver Allan McNish said.
BBC F1 technical analyst Gary Anderson said: "Red Bull were running a low downforce set-up - I think they are trying to set the car up so they can overtake people in the race a little easier. And when it is like that it is difficult in these slippery conditions.
"I was quite impressed by McLaren, when everyone was out on both the intermediates and the dry tyres, they looked quick. They're definitely more competitive here than they were in Hungary."
The forecast for the second session, which starts at 13:00 BST, is for warm, dry weather. However, rain is expected to disrupt both Saturday's qualifying session and Sunday's race.
Lotus drivers Kimi Raikkonen and Romain Grosjean both ran the team's straight-line speed-boosting device, which reduces drag by 'stalling' the rear wing automatically above a specified speed.
The intention is to race the system, but the team will not make a final decision on that until Friday evening.